In many areas of the world, surface water is unavailable for much of the year, and ground water has high concentrations of dissolved ions. WHO recommends total dissolved solids (TDS) which consist of dissolved salts and silica in drinking water to be not more than 500 ppm, however in many parts of the world, the population has no recourse but to drink groundwater with TDS concentrations as high as 800 ppm, 1000 ppm, and even in some cases 3000 ppm.
Methods to reduce ionic concentrations to more palatable levels include distillation, solar distillation, reverse osmosis, and ion exchange. In many cases these are not considered affordable, practical, or effective (e.g., owing to lack of adequate capital, land surface, solar insolation, or energy access). For years, membranes and ion exchange have been used to lower TDS from water and wastewater. These methods are not economical, practical or efficient for drinking water treatment, as they require high capital investment, substantive maintenance, large scale engineering, and energy inputs, and/or require use of toxic and corrosive chemicals.
The basic concept for separating compounds that are dissolved in water using electrical mean is quite old—dating to 1950. The technology began to be refined starting in 1990. Capacitive Deionization (CDI) is a process that applies a direct current electrical bias across a pair of electrodes immersed in the aqueous electrolyte to separate positively charged cations and negatively charged anions via migration and physical adsorption at the electrodes. For instance, in saline waters the positively charged sodium migrates to the negative electrodes and the negatively charged chloride ion migrates to the positive electrode.
The Electronic Water Purifier (EWP) is a new technology developed in the last 10 years that has low operating costs, low rejection wastewater volume, low capital expenditure, no chemical requirements, a small footprint and is now available in sizes ranging from under-the-sink water purifiers to large commercial units. EWP uses a CDI process to remove dissolved ions from water and utilizes a semi-permeable, ion-specific membrane to increase salt capture efficiency of the electrodes. The device consists of multiple layers including current collectors, conductive high surface area electrodes, and an ion-specific membrane. An electrically non-conductive spacer material separates the plates from one another, while allowing a flow of water between the electrodes, parallel to the electrode surfaces.
In common practice, these electrodes are alternately connected to the two sides of a DC power supply. The device works on the principle of capacitive deionization to purify water, with the application of a low voltage DC potential to attract and discharge ions to the electrode surface. The high-surface-area carbon electrode layers attract and hold ions on their surface removing them from the water stream flowing through the inter-electrode gaps. After some time interval of such operation, all the charged sites are filled, and the device must then be regenerated by discharging the ions from the electrode surfaces. This is accomplished by shorting the electrodes or reversing the polarity of the applied DC potential. Once a substantial number of the newly displaced ions are flushed in the waste stream, after a length of time, the unit begins to charge again by attracting ions from the feed solution under the influence of the reverse potential.
To prevent ions just crossing over to the other electrode when the voltage is reversed sometimes called the ping-pong effect), it is customary to add an ion-selective membrane on top of each electrode. The membranes increase cost, add risk of fouling, but improve short term performance.
However, this method is still limited by a relatively slow mass transfer across the electrolyte and inadequate ion storage capacity and relatively high operation cost. Currently, there are no inexpensive, energy-efficient and water-efficient methods to reduce ionic salts appearing as total dissolved solids (TDS) in water sources.
Treatment of low-TDS water sources has wide ranging applications and includes municipal, agricultural, and industrial sectors among others.
Most of the CDI devices thus far available commercially use carbonaceous electrodes. Activated carbons, carbon aerogels, and carbon nanotubes offer high surface active area and relatively large ion storage capacity. However, such electrodes are often brittle and difficult to handle, their dense nanoporous structure is hardly accessible to solutions which impedes rapid mass transfer, and some of these nanoengineered materials are prohibitively expensive. The high surface areas of these materials is also prone to fouling and degradation. Additionally, expensive ion-selective membranes used in all commercial CDI devices add cost and maintenance challenges.